In general ionising radiation is considered to be radiation within the energy range 5 KeV to 6 MeV and includes gamma rays, x-rays, beta-rays, alpha-rays and neutron beams. Devices for detecting ionising radiation are well-known for radiological protection and metrology, such as in health or nuclear physics as well as national/homeland security and anti-terrorist applications. The devices are one of two types, either passive detectors or electronic-based active detectors.
Passive detection systems use film (film-badges), thermo-luminescent detection (TLD) or photochromatic technologies (PC) as detector materials. Common to these detector technologies is that they register the presence of ionising radiation by a change of state. For example, a film exposed to ionising radiation goes dark when developed, TLD materials emit light when heated having previously been exposed to ionising radiation and PC materials change colour when irradiated with ionising radiation. However, the change of state of these materials requires special processing in order to be determined, for example developing the film or heating the TLD material. Consequently, only an historic monitoring and evaluation of radiation exposure can be obtained. It is not possible to achieve real-time monitoring and evaluation. Since no direct real-time monitoring or analysis is possible it is therefore necessary to infer what type of radiation exposure caused the change of state. Although such inference can be drawn based on experience, nevertheless it is not possible to precisely determine what type of radiation (spectroscopic information) has been sensed nor an estimate of radiation dose which takes into account such information. Additionally, known passive detection systems generally have poor sensitivity to ionising radiation.
Active detectors may be based upon silicon technology and generally comprise one, two or three PIN-diodes, each PIN-diode having a preset threshold level to signal an alarm relating to a minimum energy level of incident radiation.
If more than one PIN-diode is used then different threshold levels may be preset corresponding to different radiation and energy levels thereby providing crude spectroscopic analysis of incident radiation. However, silicon has poor sensitivity to ionising radiation since it does not have a high atomic number (Z), therefore there is inefficient conversion of the incident radiation to electric current and devices using such technology suffer from poor signal to noise ratio.
Another drawback of known active detectors is that the electronic signals are generated remote from the detector substrate, leading to signal losses and signal mis-shaping due to the impedance of connecting wires and circuitry.
In order for accurate measurement of absolute values such as the amount of exposure to incident radiation (dose), the rate of that exposure (dose rate) and the type of radiation exposure (radiation isotope) it is important that the charge value collected by an electrode, e.g. each pixel contact pad, is properly representative of the energy of the incident radiation giving rise to a photon-interaction event which generates the charge. However, The Applicant has appreciated that errors may be introduced into the measurement of charge by the very nature of the radiation that is being measured, and not only due to errors and noise in the detectors themselves.
One example is where the radiation is incident at the mid point between two pixels, which is likely, then a photon interaction event may cause charge to propagate towards two adjacent pixels in the same frame. This would be recorded as two separate hits. However, a lower energy hit would be measured than that which actually occurred because the energy of the event has been spread across two pixels rather than one. Note, for the purpose of this description a “hit” means charge collected from a single photon interaction event, plural hits corresponding to plural photon interaction events.
Other examples of causes of error are where:                (a) two hits in the same pixel in a single frame are recorded. This would result in the measurement of a higher energy hit than really occurred since the summation of the two hits within the pixel would look like a single higher energy hit.        (b) two hits in adjacent pixels in the same frame are recorded. This is the effect that “spatial correction” assumes will not happen. Such events are summed together via “spatial correction”. An error condition would be present in the presence of two actual adjacent photon hits. Spatial correction would cause a higher energy hit than really occurred to be measured, since the two hits will be combined as though they were two interactions from the same energy hit.        (c) two hits in the same pixel in successive frames cause a problem as they may be separate hits corresponding to separate interaction events. For such a situation, fixed pattern noise removal using frame differencing will result in the difference frame recording the difference in the hits from successive frames, rather than “hit minus no-hit”.        
Embodiments of the present invention were devised with the foregoing in mind.